174 WEATHER AND FORECASTING VOLUME 15

Modeling Coastally Trapped Wind Surges over Southeastern . Part II: Intensity and Depth

HELEN J. REID School of Mathematics, University of , , Australia

(Manuscript received 17 May 1999, in ®nal form 3 November 1999)

ABSTRACT A numerical weather prediction (NWP) model at the School of Mathematics, University of New South Wales, has been used to simulate the southerly buster, a southerly wind surge along the coast of New South Wales (NSW), which occurs during the spring and summer months. Three southerly buster events were simulated and comparison of the model results with observational data demonstrated that the NWP model was very good in simulating this type of event. These simulations were then used to consider the intensity, depth, and location of the southerly buster surges as they progressed northward. Both the strength and depth of the southerly buster surge decreased as it progressed farther north, particularly at the Hunter Valley. However, north of the Hunter Valley the intensity and depth increased again (in two of the three cases in this study) before dissipating completely. The location of the central part of the jet is adjacent to the , and the central jet splits at the northern side of the Hunter Valley to ¯ow around either side of the mountains. The southerly ¯ow into the Hunter Valley generally occurs before the continuation of the southerly buster up the coast. Sensitivity experiments, in which the topography at and north of the Hunter Valley was altered, were designed so that the southerly buster surge was able to develop naturally and then encounter different topographical features. These experiments indicate that the southerly buster is weaker and slower due to the natural break in the mountain barrier at the Hunter Valley and that the mountains to the north of the Hunter Valley act to delay the movement farther north with indications of reintensi®cation in the region. It is concluded that the topography has a signi®cant effect on the propagation of the southerly buster.

1. Introduction signi®cant consequences, but there is little documen- tation about this. The questions of how far north the The southerly buster is a strong, sudden, squally, southerly buster goes, and whether or not it reach Coffs southerly wind surge along the New South Wales Harbour, north of the Hunter Valley, are of great interest (NSW) coast in spring and summer. There have been for various reasons, for example, relief from hot weather many studies (see, e.g., McInnes and McBride 1993) on and planning alternatives for weather-dependent activ- the development of the southerly buster but few on the ities. Likewise, the idea of a split in the surge with the propagation farther north. A closer look at the intensity cool, southerly wind ¯owing up the Hunter Valley may and depth of the southerly buster would also provide answer in part the question of how far inland does the useful information to assist the dif®cult problem of fore- southerly buster penetrate. The split ¯ow is of interest casting these surges. Intensity is a very important aspect to those in the Hunter Valley, much as it is for those of the surge as it is the wind strength that is felt and on the coast north of the Hunter Valley. causes the damage. An indication of intensity prior to An overview of southerly busters is presented in sec- a southerly buster event would enable precautionary tion 2 and three case studies have been included in measures to be taken to minimize damage. The depth section 3. The 27 February 1998 event is covered in of the surge is also of interest, particularly to aircraft more detail and is of a more ``classical'' nature than the potentially ascending or descending through the tur- events of 18 January 1998, when the southerly buster bulent surge. It is well known that the southerly buster did not penetrate as far north as Coffs Harbour, and of is trapped against the Great Dividing Range and the 28 November 1997 with the southerly buster occurring break in the barrier at the Hunter Valley would have earlier in the day resulting in a warmer atmosphere for the surge to ¯ow into on the north coast. The high-resolution (HIRES) numerical weather pre- Corresponding author address: Helen J. Reid, School of Mathe- diction (NWP) model used to simulate the case studies matics, UNSW, Sydney 2052, Australia. is detailed in section 4. Section 5 details model veri®- E-mail: [email protected] cation for the three case studies with the simulated re-

᭧ 2000 American Meteorological Society

Unauthenticated | Downloaded 10/01/21 02:09 AM UTC APRIL 2000 REID 175 sults for intensity, depth, location, and split at the Hunter Valley in section 6. Sensitivity experiments involving the topography to consider the effect of the orientation and location of the mountain barrier on the impinging surge are contained in section 7. Section 8 presents our conclusions.

2. The southerly buster of southeast Australia Southerly busters occur during the spring and summer months in the southeast of Australia along the eastern side of the Great Dividing Range. They are a strong, sudden and squally southerly wind surge of at least 15 msϪ1 and have been known to gust up to 35 m sϪ1 (Colquhoun et al. 1985), and are con®ned to the coastal regions, trapped against the mountains by the Coriolis force. The depth of the surge is generally less than 1 km and therefore is below the average height of the Great Dividing Range. The passage of the southerly buster is occasionally accompanied by a roll cloud and is notable, not only for the common wind shift from northwesterly to southerly, but also for the sudden drop of temperature that can be up to 15ЊC within minutes (Colquhoun et al. 1985). A signi®cant rise in the sea level pressure (SLP) occurs as a ridge of high pressure follows the cold frontal system but precipitation is not FIG. 1. A map of southeast Australia over the domain of the 30- usual. km resolution model simulation indicating the location of the Great A southerly buster is generated when a cold front is Dividing Range (shaded region over 500 m), New South Wales, Vic- blocked and experiences anticyclonic deformation near toria, Queensland, the Hunter Valley, , and the . the southern parts of the Great Dividing Range in Vic- toria (McInnes 1993). The surge of air propagates north- Scone, Taree, and Coffs Harbour. All these locations are ward as a coastally trapped orographic jet up the east on the coast of NSW, except Cessnock and Scone, which coast of Australia, the duration usually being about 24 are in the Hunter Valley. Figure 2 illustrates the loca- h (Baines 1980), from the time the cold front reaches tions of these places; they are referred to in order of the Great Dividing Range to its dissipation on the north northward progression from 1 through 8. coast of NSW, or on the southern coast of Queensland For the three case studies presented, a broadscale syn- (McInnes and McBride 1993). See Fig. 1 for the lo- optic chart is included. While these are not able to show cations of these regions. the details of the southerly busters, they do provide In Reid and Leslie (1999), the focus was on the timing overviews of the synoptic situations. The time of these and speed of propagation of the southerly buster along charts is 0900. [All times referred to in this study are the NSW coast. Twenty southerly busters were chosen in Australian eastern standard time (UTC ϩ 1000), un- and HIRES demonstrated the ability to simulate the gen- less otherwise stated.] Comments on the synoptic sit- eral southerly buster ¯ow with regard to the SLP pattern uation are based on 3-hourly hand analyses from the and the characteristic S shape forming in the cold front. Australian Bureau of Meteorology, Sydney of®ce. The time of arrival of the southerly buster and the as- sociated speed of propagation were covered in more detail. Speci®c locations were chosen for the veri®cation a. The 27 February 1998 event of the simulated time of arrival of the southerly buster The event of the 27 February occurs near the end of and it was shown that the model was very good in this the southerly buster season. This particular event moved respect. The model also demonstrated skill in simulating up the southern coast of NSW during the morning to the speed of propagation. reach the region just south of Sydney at about 1300 and continued up the central coast during the afternoon, but did not reach the northern parts of the NSW coast until 3. Case studies the morning of 28 February 1998. Observational data of various forms from the Bureau Analysis charts indicate that a cold frontal system in of Meteorology have been collected for eight veri®ca- the eastern bight region at 1700 26 February 1998 con- tion locations. From south to north, these are Bellambi, tinued east until it had crossed over northeast NSW by Fort Denison, Norah Head, Williamtown, Cessnock, 1400 28 February 1998, a period of approximately 2

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FIG. 3. SLP chart for 0900 27 Feb 1998.

0800 28 February 1998. Throughout the period there was a low pressure cell over the northwest of the con- tinent. A high pressure cell to the south of Australia continued east and was to the west of at the end of the period. Details of the observations at the eight locations are included below to indicate the changes in the conditions with the passage of the southerly buster. This provides an overview of the wind, while Table 1 includes the FIG. 2. A map of New South Wales and Victoria over the domain temperature, dewpoint, relative humidity, SLP,and rain- of the 10-km resolution model simulation indicating the locations of fall. the places used for veri®cation: 1, Bellambi; 2, Sydney; 3, Norah In Bellambi the wind changed from a northwesterly Head; 4, Williamtown; 5, Cessnock; 6, Scone; 7, Taree; and 8, Coffs Harbour. The 500-m topographical contour indicates that Cessnock in the morning to a (northeasterly) around and Scone are in the Hunter Valley. noon ahead of the southerly buster, which turned the wind to a southerly at 1308 27 February 1998. The maximum speed of 19 m sϪ1 occurred at 1308, likewise days. Figure 3 shows the synoptic situation of the south- the maximum gust of 21.6 m sϪ1. erly buster at 0900 27 February 1998. In Sydney, a northwesterly turned to a sea breeze A complex cold frontal system through western Vic- (northeasterly) at 1300 27 February 1998; a southerly toria and central Australia at 2000 26 February 1998 at 1430 indicated the arrival of the southerly buster in shows signs of the S deformation through the southeast Sydney. Maximum values of wind speed and gusts were corner at 0200 27 February 1998. By 1100 27 February recorded at 1515 with values of 13.9 m sϪ1 and 22.6 m 1998, the S was more pronounced with the system on sϪ1, respectively. the NSW south coast. There was a steady northward Norah Head experienced a northwesterly before the progression of the cold front during the afternoon of 27 sea breeze (northeasterly). This sea breeze continued February 1998 before moving into northeast NSW by until 1616 when the southerly buster arrived. At Norah

TABLE 1. Observational summary for 27 February 1998.

Temp (ЊC) Dewpoint (ЊC) Relative humidity (%) Pressure (hPa) Precipitation Location Before After Before After Before After total (mm) Before After Bellambi 32.8 27.2 15.8 19.3 36 62 12 Sydney 35.5 30.7 16.4 20.7 31.9 55.2 8.6 14.5 22.1 Norah Head 24.6 20.6 19.4 20.4 73.2 87.9 15.8 16.8 18.0 Williamtown 33 19 18 19 41 100 14 22 Cessnock 36.7 27.1 5.6 20.2 26.9 66.0 0 13.2 21.0 Scone 34.0 25.9 12.4 16.7 27.0 56.8 0 15.7 22.3 Taree 21.5 19.9 19.0 17.0 86.2 77.0 1.2 19.5 23.2 Coffs Harbour 23.7 23.2 20.4 21.1 81.7 87.9 1.6 16.2 21.0

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FIG. 4. SLP chart for 0900 18 Jan 1998. FIG. 5. SLP chart for 0900 28 Nov 1997.

Head the maximum speed and gust of 20.6 and 25.2 m sϪ1 were recorded at 1700. southerly buster from propagating that far north. This A northwesterly at Williamtown the morning of 27 event occurred during the afternoon and evening in the February 1998 continued until 1500 when it was re- region of the central NSW coast. Figure 4 shows the placed by a sea breeze (easterly). However, the southerly synoptic situation of this event. Ka- buster went through Williamtown at 1720. Maximum trina is located off the northeast coast of Australia with wind speeds of 18 m sϪ1 occurred at 1830 with gust of a high pressure system in the Tasman Sea. Another high 24.7 m sϪ1. The database record for Williamtown has pressure system is located behind the trough and the no information from 1700 27 February 1998 until 0445 cold frontal system of the southerly buster, which is over 28 February 1998. The anemograph shows that the the southeast of Australia. Section 5 contains details of southerly buster occurred at 1720, just after the start of this southerly buster and the simulation for this event. the record break. During the morning of 27 February 1998 at Cessnock, c. The 28 November 1997 event the wind was a northwesterly. By 1700 27 February 1998, the wind had turned to a southerly. The maximum The 28 November 1997 event provides a different gust of 21.1 m sϪ1 occurred at 1708 27 February 1998, atmosphere ahead of the southerly buster in the northern with the maximum speeds following at 1727 of 11.3 m part of NSW. Occurring during the early morning along sϪ1. the southern part of the NSW coast, the southerly buster At Scone, once again, there was a northwesterly ahead arrived at the northern part of the NSW coast during of the southerly buster. Scone is west of the Great Di- the hotter part of the day. The synoptic situation is viding Range, in the Hunter Valley, so the southerly shown in Fig. 5. Low pressure dominates over most of buster is taken when the wind direction changes from the Australian continent and extends into a trough over a northerly to a southeasterly at 1844 27 February 1998. southeast Australia from which the southerly buster de- The maximum speed of 16.5 m sϪ1 and gust of 20.6 m veloped. A high pressure cell is west of the trough sys- sϪ1 occurred at 1850 27 February 1998. tem. Once again, the details of this event are in section 5. In the early morning of the 28 February 1998 there was a northwesterly at Taree. When the southerly buster 4. HIRES arrived at 0500 28 February 1998, the gusts were 10.8 msϪ1. It is noted that the air was already moist from a Most of the details of HIRES, the NWP model used sea breeze, and, therefore, the dewpoint and relative for this study, are listed in Leslie et al. (1985) with humidity dropped slightly with the change of air mass. modi®cations as in Reid and Leslie (1999). The model Over the night of 27 February 1998, the wind dropped uses the advective form of the primitive equations for to almost calm at Coffs Harbour until it turned south- momentum, mass, moisture, and thermal energy with westerly at around 0700 28 February 1998. The wind integrations carried out on the staggered Arakawa C grid speed rose to 10.4 m sϪ1 at 1100, gusting to 16.5 m sϪ1. using a split semi-implicit time-differencing scheme. Sigma coordinates are used in the vertical. The surface layer is parameterized by the Mellor±Yamada level-2.25 b. The 18 January 1998 event surface scheme. The rest of the boundary layer is treated On 18 January 1998, a strong sea breeze on the north as stability dependent with eddy difusivities functions coast of NSW, notably at Coffs Harbour, prevented the of the bulk Richardson number. Other features include

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TABLE 2. Outline of the error bands for time, direction, and speed TABLE 3. Summary of the arrival time and the wind direction at when comparing the simulations to the observations. that time, together with a measurement of skill, at each location for 27 Feb 1998. Excellent Very good Good Fair Poor Rating (ex) (vg) (g) (f) (p) Observation Simulation Skill Time (h) 0±1 1±2 2±3 3±4 4 ϩ Arrival Time Local Direc- Direc- Direc- Direction (Њ) 0±15 15±30 30±45 45±60 60 ϩ Place time tion (Њ) Time tion (Њ) Time tion (Њ) Speed (m sϪ1) 0±2 2±4 4±6 6±8 8 ϩ Bellambi 1308 180 1500 188.8 vg ex Sydney 1430 190 1700 184.8 g ex Norah Head 1616 200 1900 184.8 g ex Willliamtown 1820 200 2100 192.2 g ex a surface heat budget with prognostic equation for sur- Cessnock 1600 220 2000 179.9 p g face temperature, and use of the modi®ed Kuo scheme Scone 1844 140 2200 166.0 f vg Taree 0500 170 0000 224.0 p f for convection. Large-scale precipitation as well as Coffs Harbour 1026 160 0500 205.2 p p weekly average sea surface temperature are used. For the cases used in this study, the initial ®elds for the simulation were provided by the Limited Area Predic- tion System (LAPS; Puri et al. 1992) ®les, with 75-km lations. The results of these simulations of 10-km res- resolution over the whole of Australia. The HIRES sim- olution are the basis of this study. ulations were forecasts because only the operational data available at the time and the boundary conditions from 5. Model veri®cation the operational LAPS model were used. The HIRES simulations were nested in these datasets The NWP model used to simulate the southerly buster (using biquadratic interpolation) and run for 48 h with has been outlined in section 4. The skill of the simu- a resolution of 30 km (0.3Њ) over the domain of NSW, lation is considered in absolute terms, as outlined in Victoria, and Tasmania, that is, 45Њ±25ЊS and 140Њ± Reid and Leslie (1999). Once again the time of arrival 160ЊE. This domain is illustrated in Fig. 1. There were is rated as excellent to poor, as described in Table 2, to 20 sigma levels for the simulations: 0.10, 0.20, 0.30, re¯ect a difference between observations and simulation 0.40, 0.50, 0.60, 0.70, 0.75, 0.8, 0.825, 0.85, 0.875, of less than 1 h to more than 4 h. Similar principles of 0.90, 0.925, 0.95, 0.975, 0.99, 0.995, 0.998, and 0.999. measuring skill have also been applied to direction and A greater number of levels in the lower atmosphere than wind speed and are also listed in Table 2. The general is often the case were used to accommodate the very performance of this particular model in simulating the nature of the southerly buster as a shallow phenomenon southerly buster is good to very good in both time of and to assist in the determination of its height. The grid arrival and speed of propagation, details of which are size for this domain and resolution is 69 ϫ 67 and over contained in Reid and Leslie (1999). the 20 levels the model run took approximately 15 min on a high-performance workstation. a. The 27 February 1998 event Simulations of 10-km (0.1Њ) resolution were nested in the 30-km runs described above with the same initial 1) WIND TIMING, SPEED, AND DIRECTION time and duration over the slightly smaller domain of There are several aspects to consider when determin- the eastern parts of NSW and Victoria: 40Њ±29ЊS and ing how well HIRES simulated the southerly buster. 145Њ±155ЊE (see Fig. 2 for domain covered). The same Table 3 summarizes the time of arrival and direction 20 sigma levels were used for these simulations. This associated with the event of 27 February 1998. Prior to smaller domain and higher resolution yielded a grid of the southerly buster event, the synoptic wind was a 101 ϫ 110 with 20 vertical levels, which took approx- steady northwesterly, which the model was able to rep- imately 80 min to run on the same high-performance resent very well and which continued to indicate a north- workstation. westerly ¯ow until the passage of the southerly buster. The initial times of the three simulations were as However, just before the arrival of the southerly buster follows: a gentle sea breeze was able to develop along the coast R case 1, 27 February 1998Ð1100 UTC 26 February for a period of only2hatBellambi, through to more 1998; than6hatWilliamtown. It is possible that the sea breeze R case 2, 18 January 1998Ð1100 UTC 17 January was not resolved either due to the resolution of the 1998; and model (Buckley and Leslie 1999, manuscript submitted R case 3, 28 November 1997Ð2300 UTC 26 November to Int. J. Climatol.) or the short duration of the local 1997. wind ahead of the southerly buster. The time of arrival of the southerly buster at the var- These initial times gave a suf®cient lead ahead of the ious locations was well simulated by HIRES for 27 southerly buster to simulate some of the developmental February 1998. For southern coastal locations as far stages of the buster from the initial time of the simu- north as Williamtown, the simulation of the arrival time

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TABLE 4. Summary for each location of the time of the maximum intensity, the maximum intensity, and the direction at that time for 27 Feb 1998. Also included is a measure of model skill. Observation Simulation Skill Intensity Speed Gust Speed Place Local time Dir (Њ) (m sϪ1) (m sϪ1) Time Dir (Њ) (m sϪ1) Time Dir Speed Bellambi 1308 180 19.0 21.6 1500 188.8 16.4 vg ex vg Sydney 1515 190 13.9 22.6 1700 184.5 17.6 g ex vg Norah Head 1700 180 20.6 25.2 2000 178.4 14.8 g ex g Williamtown 1930 180 18.0 24.7 2200 180.5 15.2 g ex vg Cessnock 1708 180 8.2 21.1 2100 171.9 14.0 f ex g Scone 1850 160 16.5 20.6 2300 142.2 11.9 f vg g Taree 0500 170 7.2 10.8 0100 207.2 12.3 f g g Coffs Harbour 1026 160 8.2 14.9 1000 195.5 11.9 ex g ex is consistently later than the observations by less than of the southerly buster. It should be noted that the record 3 h and it is considered that the model was able to at Williamtown was incomplete and the break in data simulate the speed of propagation very well for this occurs at the time of the southerly buster, which is un- region. However, for the other locations farther north, fortunate as a direct comparison between the observa- the simulation is 4±5 h later than observations. This still tions and the simulation is made more dif®cult. indicates a good propagation speed overall. The model With respect to the absolute temperature of 27 Feb- was excellent in simulating the wind direction south of ruary 1998, the model is very good in simulating the the Hunter Valley and was still good to the north of magnitude of the maximum when it is not affected by Williamtown. a sea breeze. It was noted earlier that the sea breeze Also to be considered is the strength of the southerly circulation was not resolved in this simulation. buster as wind speeds are to be greater than 15 m sϪ1. The observations indicate a temperature drop occurs The observed times of the maximum wind speeds and with the arrival of the southerly wind and the simulation gusts have been recorded and outlined in Table 4, along represents this aspect. This sudden decrease is more with the directions of the wind at that time. The max- signi®cant at the locations where the sea breeze was not imum speeds for both the observations and simulations present. The timing of the simulated temperature drop generally occurred at the time of or approximately an is later than the observed one, but only to the same hour after the arrival of the southerly buster. The sim- degree as the late simulation of the wind surge. On ulated speeds are magnitudes, obtained using both the several occasions the simulated temperature drops more north±south and east±west components of the wind. quickly than the observed temperature. Table 5 provides Overall the model was very good at forecasting the the temperature details of the locations. It should be maximum speed timing, the direction of the wind at that noted that in the observational data there is a drop in time, and also the intensity of the wind maximum. Ob- temperature at 2200 27 February 1998 at Taree, which servations indicate the direction to be southerly (be- corresponds to a signi®cant increase in relative humid- tween 170Њ and 190Њ) at the coastal locations south of ity, but the drop referred to in Table 5 coincides with the Hunter Valley, which coincides with a barrier of the southerly wind change. roughly north±south orientation. The simulation is able Table 6 compares the SLP observations and simula- to represent this very well. North of the Hunter Valley tions for 27 February 1998. Included are the minimum at Cessnock, Scone, Taree, and Coffs Harbour the wind pressure, the time at which this occurred, and the rise has a slight easterly component (between 160Њ and in pressure over the 5-h period that covers the greatest 170Њ). However, the simulation indicates a greater east- pressure change associated with the southerly buster. erly component at Cessnock and Scone (between 140Њ and 175Њ) and a slight westerly component at Taree and The time of minimum SLP is well simulated at all lo- Coffs Harbour (between 190Њ and 210Њ). The compar- cations. Note that after 5 h, the simulated pressure rise ison of the wind speed indicates that differences between is signi®cantly less than the observed rise and so the observations and the simulation are generally less than simulated SLP rise was also calculated over a period of 5msϪ1. This comparison was made using the observed 10 h. This extended time for the simulated SLP rise average maximum wind speeds and not wind gusts. provides results that are closer representations of the observed 5-h rise. For the locations south of the Hunter Valley, the sim- 2) TEMPERATURE AND SLP ulation provides a good representation of the southerly Having already considered the ability of the NWP buster for the wind time, speed, direction, and temper- model to simulate the wind of the southerly buster, we ature. On the coast to the north of the Hunter Valley next consider the temperature and sea level pressure the simulation was fast, resulting in early arrival times, ®elds, which are also signi®cantly affected by the arrival yet in the Hunter Valley itself the simulation was slow,

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TABLE 5. The time of maximum temperature, maximum temperature, and the decrease associated with the southerly buster for each location for 27 Feb 1998. Observation Simulation Temp (ЊC) Place Local time Max Drop Time Max Drop Bellambi 1254 32.8 6.1 1400 34.0 11.8 Sydney 1400 35.7 11.2 1600 34.3 10.3 Norah Head 1615 24.8 2.0 1700 31.7 8.3 Williamtown 1700 33.0 Ð 1800 30.6 7.2 Cessnock 1700 36.7 9.6 1900 29.9 8.0 Scone 1844 30.1 4.2 2100 26.9 7.4 Taree 0500 21.5 1.6 2300 29.0 4.8 Coffs Harbour 1000 25.2 1.9 0400 26.0 3.7 with late arrival times. The temperature decrease as- error of only 2 or 3 m sϪ1. A comparison between the sociated with the southerly buster is also well timed and observed and simulated wind directions shows that for the magnitude of this drop was good in the absence of both the initial direction and direction of the maximum a sea breeze. While the simulated SLP rise is not as speed there is a difference of less than 15Њ at most of rapid as the observed one, the time of the occurrence the locations. For the majority of the locations the tem- and the magnitude is very good. However, overall this perature drop was well simulated by the model but the is not a good representation of changes at the Hunter observed SLP rise was higher than the simulated rise. Valley. Inclusion of the simulated SLP rise over a 10-h period gives a better absolute rise when compared with the observations over a 5-h period. Overall, the model per- b. The 18 January 1998 event formed extremely well for this southerly buster event. It should be noted for 18 January 1998 that once again Details of the changes associated with this case are listed there is a break in the record at Williamtown, and also in Table 7. that the southerly buster did not travel as far north as Coffs Harbour. The data for Coffs Harbour are still in- c. The 28 November 1997 event cluded so we may consider the ability of the model in that region with the nonevent of the southerly buster. Table 8 summarizes the changes associated with the The time of arrival of the southerly buster is within southerly buster of 28 November 1997. The simulated an hour for all locations, except for Bellambi and Fort time of arrival coincided with the time of the observed Denison, where there is a 1.5-h lead by the model. This maximum speed; generally it was less than 2 h after the is an excellent result as the simulation is consistently observed arrival time. The simulated wind speeds are within the error range that is useful to forecasters (Reid slightly higher than the observations and generally are and Leslie 1999). Initial wind speeds of the southerly only slightly lower than the maximum gusts. The less have been well simulated with the error generally being intense winds that were observed at the initial time of only2or3msϪ1. The maximum wind speeds occurred the observed southerly buster were not represented in approximately 1±2 h after the change to the southerly the simulation. It is concluded that the model was able direction, with the model having an error of less than to simulate only the stronger winds that occurred later 3 h at half of the locations. Simulated maximum wind in the observed surge. Direction is simulated very well speeds were compared to the observed wind gusts. This except at the Hunter Valley locations and Taree. The comparison is useful as it provides a range of wind simulated maximum temperature is approximately 6ЊC speeds that may be expected. Once again there is an too high, possibly due to an incorrect representation of

TABLE 6. As for Table 5 but with SLP for the 27 Feb 1998. Included is the minimum SLP, the SLP rise over a 5-h period for both observation and simulation, and the SLP rise over a 10-h period for the simulation. Observation Simulation Pressure (hPa) Place Local time Min 5-h rise Local time Min 5-h rise 10-h rise Bellambi 1254 Ð Ð 1400 1013.4 5.7 8.2 Sydney 1400 1014.2 6.1 1600 1014.2 5.4 6.8 Norah Head 1500 1014.5 6.5 1700 1015.6 3.8 5.0 Williamtown 1800 1014.0 Ð 1900 1016.5 3.2 4.2 Cessnock 1600 1013.2 7.8 1800 1016.2 3.3 4.9 Scone 1700 1015.7 6.1 2000 1016.5 3.9 4.5 Taree 1600 1015.1 4.4 2100 1016.7 2.0 3.8 Coffs Harbour 0330 1018.2 2.8 0300 1017.6 2.2 1.9

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the diurnal cooling in the model, and the resulting sim- ulated temperature drop is too great. Over a 5-h period after the minimum SLP occurred, the simulation has a higher value than the observed SLP rise. Overall, the ability of the model to simulate the arrival 5 3.6 time of the southerly buster was good to very good, the ഠ n Table 6) at each of intensity was also good to very good, and, likewise, the

SLP rise (hPa) direction was good to very good at the locations south of the Hunter Valley and still acceptable to the north. 0 m initial southerly wind values. ഠ

Obs Sim Sim The temperature extremes in the simulations were gen- erally not well resolved but the distinct decrease of tem- perature associated with the southerly buster was clear. The SLP rise was good for 28 November 1997, but was slow for 27 February 1998 and 18 January 1998. From

4.0 these three very different cases and their respective sim- C) ഠ Њ ulations, it is noted that there are some limitations in the model con®guration used in this study, but in sim- ulating the characteristics of the southerly buster that are more important, namely the wind variables, the mod- 4.0 2.54.72.7 1.8 5.0 5.4 Ð 6.60.5 4.5 8.8 3.5 2.0 6.8 3.3 6.8 6.7 5.2 1.9 4.6 Temp drop ( 11.011.3 11.815.7 11.6 9.1 8.0 8.9 6.2 Ð 4.8 6.2 4.6 7.1 6.4 Obs Sim ഠ el is good to very good. It is noted that despite the lack of local wind effects, such as sea breezes, in the sim- ulations the larger-scale change of the southerly buster is still good. E → ) 157.0 172.7 150.9 147.0 185.4 189.7 193.0 179.4 182.8 171.3 182.2 169.6 178.0 197.5 6. The southerly buster surge Њ The overall layout of the barrier in the form of the Great Dividing Range (illustrated in Fig. 6) is such that EN

Direction ( from 37.5Њ to 33ЊS the height is 800 m or more at 150ЊE → Ð and then from 32Њ to 29ЊS the height is 900 m at 151.5ЊE. 160 170 170 190 170 190 190 190 190 180 080 190 170 Obs Sim The signi®cant break in the barrier afforded by the Hunt- er Valley (between approximately 33Њ and 32ЊS) has a great impact on the northward and westward penetration of the southerly buster. The simulations provide useful information about the 8.5 N 9.7 4.3 9.8 5.3 9.3 14.4 15.4 13.4 16.1 10.1 15.7 11.0 14.9 14.8 )

ഠ longitudinal location, width, depth, intensity, tempera- 1 Ϫ ture, and density of the surge and the notable changes in these features across the Sydney Basin and the Hunter Valley. These features will be illustrated primarily using

Speed (m s the case of 27 February 1998. A comparison of the other 5.1 9.3 9.3 3.1 4.1 Ð 10.2 16.5 19.5 14.4 18.5 10.8 17.0 14.9 17.5

Obs Sim two cases used in this study will also be included for ഠ some of the features.

a. Longitude of surge In Fig. 7, ®ve latitudinal cross sections are shown and 1700 2100 2000 2300 2200 1000 1100 1400 1400 1700 1600 2100 1800 0000 are at the time when the surge peak value for 27 Feb- ruary 1998 was broadest. The pro®le of the topography at the particular latitude is included, indicating where Local time the strong core of the southerly buster is with respect Obs Sim 1800 2000 2200 1456 1736 1700 0500 to the Great Dividing Range. Location, intensity, and depth are provided in Fig. 7. The southerly component only is shown as the easterly component is negligible for many locations and the southerly indicates the region of interest clearly. 7. A summary of the 18 Jan 1998 southerly buster including time of arrival, wind speed, wind direction, temperature drop, and SLP rise over 5 and 10 h (as i Table 9 gives an overview of the relationship between Place the width of the southerly buster surge and the longi- ABLE CessnockSconeTaree 1745 1900 Ð T BellambiSydneyNorah Head 1230 Williamtown 1530 1630 1700 Coffs Harbour Ð Ð the locations. The timeObservation of and maximum simulation wind, are maximum abbreviated wind to speed, Obs and and wind Sim, direction respectively. are Note included that in the the southerly second buster line did for not each reach location Coffs when Harbour this for differed this fro event. tudinal location of the jet with respect to the steeper

Unauthenticated | Downloaded 10/01/21 02:09 AM UTC 182 WEATHER AND FORECASTING VOLUME 15 SLP rise (hPa) Obs Sim C) Њ Temp drop ( Obs Sim ) 197.7187.7191.3 3.0193.3 2.1173.6 14.7 5.0156.0 15.5 5.0191.7 16.7 7.5191.7 16.3 Ð 7.9 12.5 3.5 8.9 6.3 4.1 11.1 2.5 3.8 4.0 5.4 5.9 3.3 12.7 6.0 Ð 6.6 3.5 5.9 6.1 5.5 4.9 Њ Direction ( 180 190 190 170 140 150 190 150 180 Obs Sim

FIG. 6. Map illustrating the topography used by the model for the 10-km resolution simulations with contours every 100 m. Bold con- tours are for 0, 500, 1000, and 1500 m. 5.4 190 14.415.813.0 180 12.8 190 10.2 180 180 11.4 110 11.2 150 160 8. As for Table 7 but for 28 Nov 1997. ) gradients of the Great Dividing Range. The main trend 1 Ϫ to note is that the surge is close to the steep gradient ABLE

T while the central jet is close to the coast and narrow. The surge is simulated to be 2Њ±3Њ of longitude wide. Speed (m s 8.2 8.7 Obs Sim 15.9 11.8 16.7 10.3 12.3 10.8 10.8 b. Intensity and depth of surge Intensity and depth of the southerly buster surge are two of the most important aspects to consider. This has been done using latitudinal cross sections through the surge at different times to ®nd the maximum intensity

0600080009001100 3.6 1100 4.1 8.7 1600 8.7 1500 7.1 1800 4.6 12.9 and 4.1 the depth of the surge at those times. To consider the depth of the surge, the value of 12 m sϪ1 will be taken to indicate the edge of the core region and is

Local time generally the maximum wind speed across the Hunter

Obs Sim Valley and the lowest maximum value. At this stage 0900 0830 0930 1020 1626 1800 1618 1147 1843 only the ¯ow at the coast is considered.

1) THE 27 FEBRUARY 1998 EVENT The intensity of the southerly buster at various lati- tudes is summarized in Table 10. Along the southern coast of NSW wind speeds reach 20 m sϪ1, then drop Place to 15 m sϪ1 across the Sydney basin, and decrease further SydneyNorah HeadWilliamtownCessnock 0600 0624 Scone 0830 TareeCoffs 1100 Harbour 1420 1821 1127 Bellambi 0130 to 12 m sϪ1 as the southerly buster crosses the Hunter

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FIG. 7. Latitudinal (35Њ±31ЊS) cross sections between 149Њ and 154ЊE showing silhouette of mountain barrier (dashed line), and the southerly component of the wind (0±16 m sϪ1 with 6 and 12 m sϪ1 in bold) to indicate the location of the central jet with respect to topography. The height and sigma-level scales correspond to be approximately equal.

TABLE 9. At various latitudes, summary of the longitude and height Valley and continues up the coast. However there is a of the topography and the longitudinal band of the southerly buster delay in time as it moves farther up the coast past the jet for 27 Feb 1998. Barrington Tops peak of 1250 m (32ЊS) and increases Ϫ1 Topog- Topog- in strength to 14 m s . A period of 5 h elapses before Lat raphy raphy Long of jet Long of Ͼ3msϪ1 the continuation of the southerly surge up the coast to (ЊS) long (ЊE) height (m) (ЊE) (ЊE) the east of the Great Dividing Range during which time 37.0 150.0 1500 149.7±151.8 149.6±153.8 there appears to be some regeneration as it continues 36.0 150.0 1500 149.7±152.0 149.6±153.8 north as a deeper and stronger surge. 35.0 150.5 800 150.8±151.3 149.7±152.9 For the latitudes of 37Њ±32.6ЊS, the depth of the surge 34.0 150.5 1190 150.7±151.9 150.0±152.2 decreases from 1250 to 700 m, also summarized in Table 33.5 151.3 1000 151.6±151.9 150.1±152.3 10. This corresponds with the height of the continuous 33.0 151.5 800 151.5±152.3 150.0±152.5 32.5 151.5 650 151.6±152.0 150.1±152.3 coastal barrier, which decreases from ഠ1500 to 750 m. 32.4 151.3 350 151.9±152.2 149.9±152.5 The Hunter Valley between the latitudes ഠ32.5Њ±32.3ЊS 32.0 151.5 1200 151.8±152.1 150.1±152.3 and the height of the topography decreases to ഠ300 m 31.5 151.5 1050 152.7±152.9 151.7±153.3 in the region. The depth of the southerly buster is re- 31.0 152.0 1050 152.8±153.3 151.9±153.8 duced to ഠ500 m and continues to be this shallow until

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TABLE 10. Summary of intensity and depth of the surge along the TABLE 11. As for Table 10 but for 18 Jan 1998. coast at the peak time of the southerly buster at that latitude for 27 Feb 1998. Lat Max speed 12msϪ1 height (ЊS) Local time (m sϪ1) (m) Topography 37.0 1100 18 1000 Lat height Max speed 12msϪ1 36.0 1100 16 750 (ЊS) Local time (m) (m sϪ1) height (m) 35.0 1200 16 750 37.0 1300 1500 20 1250 34.5 1300 16 700 36.0 1400 1500 19 1000 34.0 1600 20 750 35.0 1400 800 18 800 33.5 2000 15 800 34.0 1600 1190 18 950 33.0 2200 16 800 33.5 2200 1000 16 950 32.7 2300 14 800 33.0 2200 800 15 750 32.5 0000 12 750 32.9 2200 850 15 750 32.3 0200 13 650 32.8 2200 800 15 750 32.0 0200 12 700 32.7 2300 750 15 750 31.5 0300 12 550 32.6 2300 750 14 700 31.0 0400 13 550 32.5 0100 250 12 600 32.4 0100 350 12 550 32.3 0100 450 12 500 32.2 0100 800 12 700 3) THE 28 NOVEMBER 1997 EVENT 32.1 0100 1050 12 500 32.0 0100 1250 12 500 The features of the southerly buster of 28 November 31.8 0100 950 12 400 1997 are quite similar to those of 27 February 1998. 31.5 0600 1050 13 750 The wind strengths do decrease as the surge travels 31.0 0600 1000 14 750 north, as does the depth (see Table 12). They both in- crease as the surge moves north of the Hunter Valley region. The observed depth, also recorded by AMDAR data 31.5ЊS when the heightened topography, 1200 m, at from Sydney airport, has the southerly buster surge 32ЊS affects the ¯ow and the depth of the surge increases depth with a peak value of ഠ800 m during 1100 and to 750 m. 1200 28 November 1997. The simulation has a depth According to Aircraft Meteorological Data Relay of 700 m for 34ЊS at 0900 28 November 1997. While (AMDAR) data recorded by airplanes ascending and the simulation has a slightly lower surge depth and is descending at Sydney airport (33.91ЊS, 151.16ЊE), the 2 h earlier than the observations, this is still a good maximum height of the southerly surge occurs between representation of this aspect of the southerly buster. the times of 1530 and 1630 27 February 1998 and it is The southerly buster of 18 January 1998 is the stron- approximately 850 m deep. Comparing this to the sim- gest and deepest at 34ЊS, followed by 27 February 1998, ulated time and height of the surge peak taken at 34ЊS, which is slightly stronger than 28 November 1997 at the time is 1600 27 February 1998 and the depth is this latitude. The AMDAR data for the three cases in- approximately 950 m. This indicates the model provides dicates the surges are similar in depth, with only a 100-m a very good representation of the surge for this event. difference, and the 18 January 1998 surge is the deepest. The simulations have a depth difference of 250 m over- all at this latitude with the 27 February 1998 surge 2) THE 18 JANUARY 1998 EVENT having the greatest depth. The 28 November 1997 event is the shallowest case for both the observations and the The surge of 18 January 1998 is slightly stronger and deeper as detailed in Table 11. The surge decreases with TABLE 12. As for Table 10 but for 28 Nov 1997. height according to the topography. However, in this case, the surge does not deepen again, nor does the Lat Local time Max speed 12msϪ1 height intensity of the wind increase in the region to the north (ЊS) (UTC ϩ 1000) (m sϪ1) (m) of the Hunter Valley. 37.0 0600 17 1100 Once again, AMDAR data from Sydney has been 36.0 0700 17 950 35.0 0700 17 800 used to help determine the time and depth of the surge 34.5 0800 17 800 peak at 34ЊS. The observations indicate the peak of the 34.0 0900 17 700 surge to occur between 1600 and 1700 18 January 1998 33.5 1000 14 650 and this is simulated at 1600 18 January 1998. The surge 33.0 1200 14 500 32.7 1300 12 450 depth at this time is observed as being approximately 32.5 1500 13 350 900 m, which is compared to the simulated depth of 32.3 1600 12 450 750 m. It is seen once again that the model is very good 32.0 1700 13 500 at simulating the peak time and depth of the southerly 31.5 1900 14 500 buster. 31.0 2000 14 600

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TABLE 13. An indication of the longitudinal location of the south- TABLE 14. Summary of the Hunter Valley ¯ow for 27 Feb 1998, erly ¯ow with respect to the location of the Hunter Valley, similar similar to Table 10. to Table 9 but for 27 Feb 1998. Note that the topography height indicates the valley depression. Lat Topography Max speed 12msϪ1 (ЊS) Local time height (m) (m sϪ1) height (m) Long of 32.6 0000 180 15 400 Lat Topography Topography Long of jet Ͼ3msϪ1 32.5 0100 650 14 500 (ЊS) long (ЊE) height (m) (ЊE) (ЊE) 32.4 0100 550 13 600 32.4 150.0 550 150.7±151.0 149.9±152.5 32.3 0100 500 13 650 32.3 149.7±151.3 500±300±500 150.5±151.0 150.1±152.7 32.2 0100 500±300 12 450 32.2 149.5±151.0 500±300±750 150.7±151.2 150.0±152.4 32.1 0100 500±250 12 400 32.1 149.5±151.3 500±300±1050 150.7±151.3 150.3±152.7 32.0 0100 500±300 12 300 32.0 150.8±151.5 500±300±1200 150.7±151.0 150.1±152.3 31.8 0100 900±550 12 300 31.8 150.0±151.5 900±500±950 150.3±150.9 150.1±152.9 31.5 0600 600±300 9 450* 31.5 149.6±151.0 600±300±1050 150.0±150.5 149.7±150.7 31.0 0600 450±300 7 450* 31.0 149.5±151.0 500±300±1050 149.7±150.4 149.7±150.4 * The height of the maximum speed rather than the 12 m sϪ1 height. simulations. In each of the three cases the model was as it moves up the Hunter Valley. The time of the ¯ow able to simulate the depth of the coastal part of the surge up the Hunter Valley is the same as that of the coast. very well. Table 14 includes the strength and depth of the ¯ow up The three southerly buster events all have a similar the Hunter Valley. intensity and depth at their southern latitudes and de- crease in stages to a strength of 12 m sϪ1 at approxi- mately 32.6ЊS. The surges do not decrease to a uniform 2) THE 18 JANUARY 1998 EVENT depth with the January event being approximately twice Table 15 summarizes the intensity and depth of ¯ow the depth of the November event across the Hunter Val- up the Hunter Valley for 18 January 1998. The wind ley at ഠ32.4ЊS, while the February event is between the speeds up the Hunter Valley are actually stronger (ഠ3 two. All three southerly buster surges are well simulated msϪ1) than those along the coast (cf. with Table 11). in depth and timing when veri®ed with AMDAR re- The southerly ¯ow into the Hunter Valley occurs 2 h cords, although the November surge simulation has a before the ¯ow along the coast north of the Hunter peak 2 h before the observed peak. Valley. It is noted once again that the southerly buster does not reach Coffs Harbour on this occasion. The c. Split of the surge at the Hunter Valley surge up the Hunter Valley is very deep, ഠ750 m, which is the same as the coastal surge. It is possible that the The Hunter Valley provides a signi®cant break in the sea breeze prevented coastal penetration and therefore mountain barrier of the Great Dividing Range as noted most of the surge was directed up the Hunter Valley. earlier. North of the Hunter Valley, the Great Dividing This case from 18 January 1998 is deeper overall, and Range lies farther east than it does to the south of the therefore the greater depth of the ¯ow up the Hunter valley. This acts to channel part of the ¯ow up into the Valley re¯ects the initial depth of the surge. Hunter Valley. The intensity and depth of this branch of the ¯ow are detailed below for the three cases. 3) THE 28 NOVEMBER 1997 EVENT For the case of 28 November 1997, wind speeds in 1) THE 27 FEBRUARY 1998 EVENT the Hunter Valley are similar to those along the coast. Figure 7 illustrates the central part of the surge of 27 These are summarized in Table 16. The depth of the February 1998, at southern latitudes with a similar lon- gitude to that of the mountains to the north of the Hunter Valley (at ഠ32ЊS). At the northern latitudes a strong TABLE 15. As for Table 14 but for 18 Jan 1998. south-southeasterly ¯ow is located to the west of the Lat 12msϪ1 Great Dividing Range while a southerly ¯ow still exists (ЊS) Local time Max speed (m sϪ1) heights (m) at the coast to the east of the Great Dividing Range. 32.6 2200 19 550 The strength of this secondary ¯ow in the Hunter Valley, 32.5 2300 18 550 while still quite strong, does not penetrate as far north 32.4 2300 17 800 before losing intensity and dissipating. Table 13 indi- 32.3 2300 16 850 32.2 2300 16 800 cates the longitude and depth of the Hunter Valley at 32.1 2300 15 750 various latitudes for 27 February 1998. 32.0 0000 16 750 The ¯ow up the Hunter Valley is distinct and quite 31.8 0100 17 700 strong, ഠ12 m sϪ1, decreasing to 7 m sϪ1 by 31ЊS. The 31.5 0100 16 650 31.0 0200 13 550 depth of this branch of the southerly buster is ഠ400 m

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TABLE 16. As for Table 14 but for 28 Nov 1997. in the 30-km resolution simulation over the standard Lat Local time Max speed NSW topography. The differences between the topog- (ЊS) (UTC ϩ 1000) (m sϪ1) 12msϪ1 height (m) raphy ®les are as follows: 32.6 2200 14 450 R New South Wales (NSW)Ðthe standard topography 32.5 2300 14 700 32.4 2300 14 700 over the domain 40Њ±29ЊS and 145Њ±155ЊE (Fig. 8a); 32.3 2300 14 700 R no Hunter Valley (NOHV)Ðthe same domain but with 32.2 2300 14 650 a continuous barrier, that is, the region between 33Њ 32.1 2300 13 600 and 32ЊS has been modi®ed to be a barrier of 800-m 32.0 0000 12 500 height (Fig. 8b); 31.8 0000 14 450 31.5 0000 16 450 R straight mountains with no break (STR)Ða continu- 31.0 0000 10 400* ous barrier with the mountains north of 32ЊS moved west, from ഠ152ЊEtoഠ150ЊE (Fig. 8c); and R straight mountains with a break (BRK)Ða straight ¯ow in the Hunter Valley is also similar in magnitude barrier as described for STR, but with a decrease in to the coastal depth. However, the ¯ow into the Hunter barrier height to 400 m between 32.8Њ and 32.3ЊS, a Valley occurs approximately 10 h after the surge has similar latitude as the Hunter Valley (Fig. 8d). moved up the coast. As illustrated in Figure 8, the coastline is not altered, The intensities and depths of the various Hunter Val- resulting in a coastal plain when the mountain barrier ley ¯ows are all quite different. The 27 February 1998 is moved west. Sydney (labeled 2), Williamtown (4), Ϫ1 has an intensity of 12 m s and is ഠ400 m deep; the Coffs Harbour (8), and a location labeled 9 (30.3ЊS, 18 January 1998 event is twice that deep at ഠ800 m 151.8ЊE) are included in Fig. 8 to serve as reference Ϫ1 with an intensity of 16 m s , while the 28 November points for Fig. 9. 1997 event is between the two others with an intensity These four topography ®les were employed to see the Ϫ1 of 14 m s and with an approximate depth of 600 m. effect they each would have on the propagation of the The time of the peak southerly ¯ow in the Hunter surge. The following points have been noted. Valley is approximately the same as at the coast for the 27 February 1998 event according to the simulation; however, the observed ¯ow up the Hunter Valley oc- a. Arrival times and wind speeds of the surge curred approximately 9 h before the surge up the coast continued. On 18 January 1998, the Hunter Valley ¯ow Using the coastal locations Bellambi, Sydney, Norah peaks approximately 2 h earlier than the coastal surge Head, Williamtown, and Coffs Harbour, the time of the and in this case the simulation is not signi®cantly dif- southerly change was determined as before for each ferent than the observations. By contrast there is an 8-h topography ®le. South of the altered topography (name- time difference for 28 November 1997 event with the ly at Bellambi and Sydney) there was no appreciable Hunter Valley ¯ow peak being very much later. In this difference in the time or the intensity of the southerly case the simulation is not consistently later or earlier wind. At Norah Head and Williamtown, locations close than the observations. to the Hunter Valley, the straightened mountain barrier cases had the southerly wind arriving almost an hour earlier and with wind speeds ഠ2msϪ1 higher (see Fig. 7. Topographical sensitivity experiments 9). However, at Coffs Harbour, well north of the Hunter The Great Dividing Range changes quite markedly Valley and relatively farther from the mountain barrier in the vicinity of the Hunter Valley (refer to Fig. 6). A in the cases of a straightened mountain barrier, the south- series of sensitivity experiments, using the 27 February erly wind occurred approximately 5 h later and with 1998 event, were run to identify the effect of the Hunter lower wind speeds. This resulted from being to the east Valley on the northward propagation of the southerly of the jet of the southerly buster, which is trapped against buster. The topography of the model domain was altered the mountain barrier. Farther west, at location 9, adja- to see the effect of the different land barriers. The south- cent to the straight mountain barrier and at the same east corner of Australia where the southerly buster de- latitude as Coffs Harbour (ഠ30ЊS), the southerly wind velops was not altered so the surge would be able to arrived approximately 1 h earlier than at Coffs Harbour develop and mature as usual before encountering a dif- and was slightly stronger. ferent topographical feature. The existing mountains to the south and north of the Hunter Valley are offset from b. Longitudinal location of the surge a straight line and so three additional topography ®les were added to the standard one of NSW with different At latitudes where the topography was not altered, alignment and heights of the mountain barrier. Each that is, south of ഠ33ЊS, the location, intensity, and width topography ®le had a resolution of 10 km and the sim- of the surge are essentially the same for each topog- ulations over the modi®ed topography were all nested raphy. This is as expected with no change to the con-

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FIG. 8. Comparison between the four topography ®les for 148Њ±153ЊE, indicating the changes to the topography north of 35ЊS for the sensitivity experiments. Contours are every 200 m with 0 and 1000 m in bold: (a) New South Wales, (b) no Hunter Valley, (c) straight barrier, and (d) straight barrier with break. Locations 2, 4, 8, and 9 are marked for reference. ditions in this region. Figure 10 contains a latitudinal (see Fig. 11). The intensity, as noted in section 7a, is cross section at 35ЊS for each topography, and at this similar throughout. The central jet is broader in the cases latitude no signi®cant difference between the simula- with a straight barrier. The secondary jet, split from the tions is seen. southerly buster at the Hunter Valley, observed in the In the region north of 33ЊS where the topography has simulation with the unchanged topography is not clear been altered, there is still a common feature in the lo- in the other runs. There is a secondary ¯ow of 9 m sϪ1 cation of the jet being centred over the coastline with southerly at 32ЊS in the case of NOHV continuing for the extent inland being limited by the mountain barrier approximately 12 h, and only 3 m sϪ1 southerly in the

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FIG. 9. A comparison between ®ve coastal locations and point 9, west of Coffs Harbour adjacent to the straight barrier (30.3ЊS, 151.8ЊE), to show the effect different topography has on the arrival time and intensity of the southerly wind. case of BRK for only 5 h. No secondary ¯ow was ob- topography. There is an exception for the last reference served in the STR case. point for the BRK case only as it got only as strong as 11msϪ1. The times of greatest intensity are similar for each c. Depth of the surge topography simulation until 32ЊS when the NSW is ear- The depth of the surge is considered in the same lier. However, by 31ЊS NSW and NOHV are signi®- manner as in section 6 and is included in Table 17. Once cantly later (5 h) in the time of the maximum wind again, the height of the 12 m sϪ1 contour is used as an strength, but at 30ЊS there is only a 2-h difference. The indicator of the depth of the surge and is a fairly con- straight barriers (STR and BRK) have stronger winds sistent measure for the duration of the surge over each farther north, 31Њ and 33ЊS, respectively, but these do

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Fig. 10. Similar to Fig. 7 but over the different topography simulations, with the same time for each, and comparing just the latitude of 35ЊS. not regenerate, whereas NSW and NOHV appear to re- 32.3ЊS, which is the lowest height for the NSW case generate in strength and depth at 31ЊS. The ®nal surge and one of the lowest for BRK, westward penetration depth is greater for NSW and NOHV with the only of the surge can be compared. signi®cant drop in depth occurring over the natural The farthest west the change penetrates with respect NSW topography. to the mountain barrier at these latitudes occurs for the case with the natural alignment of the mountains (NSW and NOHV) to a longitude of 150.4ЊE. The STR ¯ow d. Westward penetration in the region of the extends to the same longitude; however, this is still to Hunter Valley the east of the mountain barrier. This can be seen in It was noted in section 7a that in the cases of the Fig. 11. straight mountain barrier, the jet of the southerly buster A southerly ¯ow does occur over the NOHV barrier was stronger and advanced farther north than in the as the top of the surge is able to spill over on its way cases of the natural orientation of the mountain barrier. due north, although the bulk of the ¯ow is diverted to Using the different topography simulation results at the east along the barrier. There is little difference in

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FIG. 11. As for Fig. 10 but at 32.3ЊS.

TABLE 17. A comparison of the depth of ¯ow between the four different topography ®les for 27 Feb 1998.

Local time Max speed (m sϪ1) 12msϪ1 height (m) Lat (ЊS) NSW NOHV STR BRK NSW NOHV STR BRK NSW NOHV STR BRK 37 1400 1400 1400 1400 18 18 18 18 1250 1250 1250 1250 36 1500 1500 1500 1500 17 17 18 18 1000 1000 1000 1000 35 1600 1600 1600 1600 15 15 14 14 950 1000 1000 1000 34 1800 1800 1800 1800 17 15 17 17 900 900 900 900 33 2200 2200 2200 2200 15 15 18 17 700 750 800 750 32 2300 0200 0000 0000 12 12 15 13 400 700 700 700 31 0800 0700 0200 0300 13 13 15 13 700 700 600 600 30 1000 1000 0800 0900 12 13 12 11 650 650 500 450*

* For 11 m sϪ1 height.

Unauthenticated | Downloaded 10/01/21 02:09 AM UTC APRIL 2000 REID 191 magnitude and westward penetration of the wind The sensitivity experiments designed to consider the change. Only a slightly stronger and earlier southerly effect of the alignment of the Great Dividing Range wind is seen in the valley as opposed to over the ``valley about the Hunter Valley indicated that a north±south- wall'' in the case of NOHV. In the STR, the jet is con- aligned mountain barrier has stronger winds. When the ®ned completely to the east of the Great Dividing Range Hunter Valley is ®lled in to continue the barrier with with no ¯ow westward. There are very strong winds the natural line, there is a ¯ow over the barrier in the (13 m sϪ1) in the Hunter Valley (NSW), which continue offset line as the surge does not completely deviate from for many hours after the initial surge arrival. In contrast northward propagation. In the case of a straight barrier to this, there is only a very brief and weaker (10 m sϪ1) with a forced break, there is no signi®cant ¯ow through wind change in the break of the BRK topography and the break. only while the southerly buster moves past the opening. The questions posed with regard to the coastal and inland penetration have been partly answered. However, there are complex dynamics in the region of the Hunter 8. Conclusions Valley with the partial intensi®cation of the southerly Three southerly buster events have been simulated by buster in some cases. The peak of the Great Dividing the HIRES NWP model, and these results were com- Range to the north of the Hunter Valley may have a pared to observational data for various ®elds. Using data similar blocking effect on the surge as the mountains available in real time from the Bureau of Meteorology in Victoria have on the original cold front where the to initialize the simulations, these can then be classed southerly buster develops. The temperature differential as forecasts. The results of the simulations for 27 Feb- across the region also could have considerable effect on ruary 1998, 18 January 1998, and 28 November 1997 the continuation of the southerly buster, either into the all indicated that HIRES was very good at forecasting Hunter Valley or along the coast to the north. Likewise the most signi®cant ®elds associated with a southerly the forcing generated by the wider synoptic situation to buster event, namely, the time of the wind change, the the north of NSW may in¯uence the direction of the strength of the wind, and also the depth of the surge. surge to be coastal or inland. The temperature drops and SLP rises associated with the passage of a southerly buster, while not as good as Acknowledgments. The author wishes to thank Prof. the results of the wind change itself, were still able to L. M. Leslie for guidance in the writing of this manu- provide useful information regarding the changes in script; the Bureau of Meteorology, Australia, for ar- these parameters. chived data; Mr. R. Morison and Mr. S. J. M. Reid for As the southerly buster propagates north along the their technical assistance; and those who have assisted mountains of the Great Dividing Range, there is a break in the editing of this work. in the barrier height at the Hunter Valley and it was found that this has a signi®cant effect on the surge as REFERENCES it continues northward. First, it was noted that there is a split in the surge as the mountain barrier north of the Baines, P. G., 1980: The dynamics of the southerly buster. Aust. Hunter Valley is at a similar longitude to that of the Meteor. Mag., 28, 175±200. Colquhoun, J. R., D. J. Shepherd, C. E. Coulman, R. K. Smith, and southerly buster core, which directs part of the ¯ow up K. McInnes, 1985: The southerly buster of southeastern Aus- into the Hunter Valley while the rest continues along tralia: An orographically forced cold front. Mon. Wea. Rev., 113, the coast. However, the southerly buster does not always 2090±2107. penetrate into the Hunter Valley before the coast as Leslie, L. M., G. A. Mills, L. W. Logan, D. J. Gauntlett, G. A. Kelly, M. J. Manton, J. L. McGregor, and J. M. Sardie, 1985: A high might be expected. The 27 February 1998 event had a resolution primitive equations NWP model for operations and similar time of maximum intensity along the coast as research. Aust. Meteor. Mag., 33, 11±35. in the Hunter Valley, the 18 January 1998 event was 2 McInnes, K. L., 1993: Australian southerly busters. Part III: The h earlier in the Hunter Valley than along the coast (pos- physical mechanism and synoptic conditions contributing to de- sibly due to the strong sea breeze), and the 28 November velopment. Mon. Wea. Rev., 121, 3261±3281. , and J. L. McBride, 1993: Australian southerly busters. Part I: 1997 event was 8 h later in the Hunter Valley (as a result Analysis of a numerically simulated case study. Mon. Wea. Rev., of strong northwesterly winds through the Hunter Val- 121, 1904±1920. ley). Second, it was found that in all cases there was a Puri, K., N. E. Davidson, L. M. Leslie, and L. W. Logan, 1992: The decrease in depth and intensity as the surge progressed BMRC tropical limited area model. Aust. Meteor. Mag., 40, 81± 104. farther north with a deepening and strengthening oc- Reid, H. J., and L. M. Leslie, 1999: Modeling coastally trapped wind curring in the branch of the surge that ¯ows to the east surges over southeastern Australia. Part I: Timing and speed of of the Great Dividing Range north of the Hunter Valley. propagation. Wea. Forecasting, 14, 53±66.

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